This invention relates to improvements in hydraulic braking systems of the brake-by-wire type, in which under normal braking, the supply of hydraulic fluid to a wheel brake is controlled by an electrically operated actuator adapted to operate in response to an electrical signal from a pedal-operated transducer indicative of braking demand.
It is known in such a brake-by-wire system to provide a manual hydraulic back up which allows hydraulic fluid to be pushed through to the wheel brake in the event of an electrical failure. Typically, such a back-up comprises a pedal operated master cylinder connected to the brake via an isolator valve. During normal operation the isolator valve is closed so that the master cylinder and brake are isolated from each other, and hydraulic pressure is generated at the brake by the electrically operated actuator. In the event of a fault, the isolator valve is opened to connect the master cylinder to the brake to allow manual generation of hydraulic braking pressure to be applied to the brake.
In order to allow for adequate "pedal feel" it is also known in such a system to incorporate a compliance unit. The compliance unit, typically in the form of a piston working in a bore against a single biasing spring, is connected to the output of the master cylinder up-stream of the isolator valve. Operation of the brake pedal displaces hydraulic fluid from the master cylinder, which flows into the bore in the compliance unit. The fluid displaces the piston against the biasing spring until an equilibrium point is reached, i.e. where the force applied to the piston by the biasing spring balances the force applied to the piston by the brake fluid. Thus, by tailoring the characteristics of the compliance unit such as the spring force to mimic the compliance of a normal hydraulic braking system, it is possible to provide a realistic pedal force and displacement of the brake pedal during normal braking.
A problem with the use of such a compliance unit arises if there is a fault in the braking system. In this event, the isolator valve is opened to allow manual back-up, but the pedal travel required to operate the brake is excessive because of the combined compliance of the brake and the compliance unit. Nevertheless, this can be overcome to a limited extent by providing a second isolator valve between the compliance unit and the master cylinder which closes when manual back up is required.
A problem with the use of a second isolator valve arises when a fault is detected at a time when the compliance unit is at least partially full of brake fluid i.e. during or immediately after a brake application. In this event, the second isolator valve will effectively trap brake fluid in the compliance unit, leaving a reduced volume of fluid in the master cylinder circuit. This results in an undesirable increase in pedal travel during manual back-up, although it does reduce the overall compliance during back-up.
In accordance with a first aspect of the invention, we provide a compliance unit for use in an hydraulic braking system, said compliance unit being adapted to absorb hydraulic fluid expelled from the master cylinder during normal braking and, in the event that a fault in said braking system is detected, said compliance unit is adapted to return at least a part of said absorbed hydraulic fluid to said master cylinder.
The compliance unit in accordance with the invention is advantageous in that it will automatically return hydraulic fluid absorbed during normal braking to the master cylinder in the event of a fault. This reduces the required pedal travel. In effect, the compliance unit automatically decreases its compliance in the event of a fault.
In a preferred arrangement, said compliance unit comprises a first piston working in a bore, hydraulic fluid from the master cylinder entering the bore to move the piston along the bore in a first direction against a first and second biasing means, said first biasing means being adapted to apply a first biasing force to said first piston and said second biasing means being adapted to apply a second biasing force to said first piston in the event of a fault.
During normal operation, the first piston may therefore only be biased by a force substantially equal to the first biasing force. In the event of a fault, the biasing force applied to the piston may be increased by the amount of the second biasing force.
Preferably, the compliance unit is adapted so that in the event of a fault the combination of the first and second biasing forces applied to the first piston is at least equal to or greater than the maximum force that can be applied to the first piston by the fluid from the master cylinder. This is advantageous in that it ensures that substantially all the fluid is expelled from the compliance unit in the event of a failure.
The first biasing means may comprise a compression spring acting upon said first piston in a direction substantially opposed to the direction in which the force of the fluid from the master cylinder acts upon the piston. Such a spring is simple and reliable, as well as being easy to manufacture.
The bore may comprise a stepped bore having a first and second portion, said first portion being adapted to accommodate said first piston, and said second portion adapted to accommodate at least part of said second biasing means and being of larger diameter than said first portion.
The second biasing means may comprise a stepped piston having a first portion working in said first piston portion of said bore and a second piston portion of a larger diameter working in said second portion of said bore. The stepped piston may be adapted to move along said first bore in the event of a fault to interact with said first piston under the action of said second biasing force. Said second biasing force may be provided by a second spring having a spring force greater than said first spring force.
Normally, the second piston may be biased away from the first piston in the first portion of the bore under the action of a third biasing force. The third biasing force may comprise a hydraulic pressure generated in a cavity defined by a portion of the second piston and the stepped face of the bore. This hydraulic pressure acts to move the second piston against the force of the second spring (the second biasing force). Preferably, the third biasing force exceeds the second biasing force during normal brake operation, and is reduced to zero in the event of a fault.
The third biasing force is preferably generated by introducing hydraulic fluid, for example from a pump, into the cavity during normal braking. In the event of a fault, the hydraulic fluid can be pumped out, or allowed to be forced out under the action of the second spring moving the piston. This is advantageous in that it is a fail-safe arrangement which will work even in the event of electrical failure or loss of hydraulic pressure. A valve may be provided for alternatively connecting the cavity to a pressure source or pressure sink.
A throttle may be provided to limit the rate at which the fluid is expelled from the cavity. The throttle may comprise a restricted orifice in a line between the cavity and the reservoir.
A stop means may be provided in the second bore to limit the displacement of the second piston against the second spring.
The first biasing means may comprise a first spring adapted to act between an end face of the first piston and an end face of the second piston. This is advantageous in that it simplifies the required form of the stepped bore. Alternatively, it could be adapted to act upon the end face of the first piston and a fixed part of the bore.
According to a second aspect of the invention, we provide a hydraulic braking system incorporating a compliance unit in accordance with a first aspect of the invention, said hydraulic braking system comprising a master cylinder connected via an isolator valve to a wheel brake and a brake actuator being adapted to operate in response to an electrical signal from a pedal-operated transducer in turn to control the supply of hydraulic fluid to said wheel brake, said compliance unit being in permanent fluidic communication with an output of said master cylinder, and in which said isolator valve is closed during normal braking to isolate the master cylinder from the wheel brake, and in the event of a fault said isolator valve is opened to provide fluidic communication between the master cylinder and the wheel brake.
Preferably, said isolator valve comprises a normally open electrically operated valve. It may comprise a solenoid-operated valve.
The braking system incorporating one preferred arrangement of the compliance unit may operate as follows.
During normal operation of the braking system, hydraulic fluid is introduced under pressure into the cavity defined by the step in the bore and the step in the second piston.
This hydraulic fluid displaces the second piston away from the first portion of the bore against the second biasing force, effectively substantially cancelling out the second biasing force. The first piston is then free to move within the first portion of the bore against the first biasing force generated by the first spring. Fluid from the master cylinder can be absorbed into the first portion of the bore by displacing the first piston. This provides for adequate brake pedal feel by enabling pedal displacement and generating feedback force through the pedal. The master cylinder is isolated from the brakes at this stage.
In the event of a failure of the braking system or a fault, the master cylinder is connected to the brakes by opening an isolator valve between the master cylinder and the brakes. The pressure in the cavity is reduced at a rate limited by the throttle, which causes the resulting force acting across the stepped piston to cause the stepped piston to move towards the first portion of the bore. Because the second biasing force applied to the stepped piston exceeds the first biasing force, the first piston is moved to expel hydraulic fluid from the first portion of the bore, returning it to the master cylinder.
The combined force acting upon the first piston to expel the hydraulic fluid exceeds the maximum pressure that can be generated by the master cylinder, and so the piston can no longer be moved along the bore by the hydraulic fluid in this mode.
This ensures that brake pedal travel is reduced and that no fluid is lost to the compliance unit during manual braking.
There will now be described, by way of example only, an embodiment of the present invention with reference to the accompanying drawings in which: